An industrial flue gas cleaner of the sort in which the invention may be used is illustrated in FIG. 1. Dirty flue gas enters the installation through inlet manifold 10. The dirty gas is admitted into the various compartments 12 of the installation and flows upward through an array of sleeve-type or tubular filter bags 14, which are supported on the outside surfaces of cylindrical support cages 16. (See FIG. 2A.) The filter bags remove dust, soot, and other particulate matter from the gas as it passes through the filters. The clean gas then passes into and exits the installation via outlet manifold 18. Flow into and out of the individual baghouses is controlled by appropriate means such as inlet poppet dampers and outlet poppet dampers, as indicated in FIG. 1.
As further illustrated in FIGS. 1, 2A, and 2B, the filter bags are supported at their upper, open ends 20 by a tubesheet 22, which spans the entire cross section of the baghouse 12. The tubesheet 20 functions like a gasket, forming a seal around the upper ends of the filter bags and along the perimeter of the baghouse such that the baghouse is separated into distinct, upper and lower portions. Depending on the specific method of cleaning, the filter bags are arranged in either a rectangular or a circular array.
Common industry practice is to clean rectangular arrays of bags with compressed gas typically ranging in pressure from about 40 psig to about 120 psig (more or less depending on details of the specific design). A series of pulse pipes 24 extend across the baghouse, with one pulse pipe extending across each row of filter bags in the array. Each pulse pipe 24 has a series of orifices 26 extending along the bottom portion thereof, with one orifice positioned over each of the dust bags.
When compressed gas is used for cleaning, it is referred to as either "high-pressure/low-volume" or "intermediate-pressure/intermediate-volume" cleaning, depending on the characteristic pressure. High-pressure systems generally operate at a pulse pressure on the order of 80 psig to 120 psig; intermediate-pressure systems generally operate at a pulse pressure on the order of 40 psig to 60 psig.
Circular arrays of bags, on the other hand, are cleaned by gas that is pressurized with a blower to pressures typically on the order of 10 psig to 20 psig (again, more or less depending on the specific design). Because lower pressures and larger volumes of gas are used in this form of cleaning, it is referred to as "low-pressure/high-volume" cleaning.
As shown in FIG. 2A, for all but low-pressure/high-volume cleaning, during normal filtering operation, gas with entrained particulate matter enters the baghouse 12 through inlet 30 at the lower end of the baghouse. The gas flows through the filter bags 14 (which are supported on the exterior surfaces of the cages 16) from the outside in, as indicated by the schematic cross-section of the filter bag at the top of FIG. 2A. Dust, soot, ash, and other particulate matter or debris accumulates on the outside surfaces of the filter bags, and the now-clean gas exits the baghouse through the clean gas exhaust 32 at the upper portion of the baghouse.
When debris accumulates to the point that pressure drop across the bags exceeds a preset limit, i.e., where flow through the baghouse is restricted (or in many instances on a regular, timed basis), the filter bags are cleaned of debris using the pulse pipes 24. Each of the pulse pipes is supplied with pressurized gas by pressure header 34. At the appropriate time, a valve 25 is actuated and pressurized gas flows into the pulse pipe. An energetic pulse of pressurized gas flows out of the pulse pipe through each of the orifices 26 and down into the interior of each of the sleeve-type filter bags in the row, as illustrated schematically by the cross-section of the filter bag at the top of FIG. 2B. The filer bag rapidly expands to its full circumference and then stops expanding suddenly. This rapid expansion and deceleration causes the "cake" of debris which has accumulated on the filter bag to fracture and be dislodged from the filter bag. The dislodged dust cake then falls into hopper 36 at the bottom of the baghouse, where it is collected and removed by an ash removal system (not shown). (The flow of dirty gas into the compartment may be suspended during cleaning of the filter bags such that the dislodged dust and other debris settles into the hopper, rather than being blown up toward the tops of the filter bags.)
Various experiments which have been conducted by, for example, Southern Research Institute, the assignee of this application, have shown that low-pressure/high-volume pulse-jet cleaning is generally superior to high-pressure/low-volume and intermediate-pressure/intermediate-volume pulse-jet cleaning. In low-pressure/high-volume pulse-jet cleaning, a blower is used to supply only moderately compressed air for the cleaning, in contrast to a high-pressure or intermediate-pressure header as shown in FIGS. 1, 2A, and 2B. Because a blower is required to supply the relatively large volume of air utilized in this form of cleaning, it generally has been conceded by those skilled in the art that multiple blowers would be required in order to apply this type of cleaning to filter bags arranged in the more conventional square or rectangular array, as they are arranged in high-pressure/low-volume and intermediate-pressure/intermediate-volume pulse-jet cleaning systems.
Providing multiple blowers, however, is not economical. Accordingly, low-pressure/high-volume pulse-jet cleaning has only been able to be realized on a commercial, practical scale by arranging the filter bags in concentric circles and supplying the pulses of air to the filter bags by means of a rotating arm. The arm rotates about an axis that is centered in the middle of the concentric circles of filter bags and is supplied with air through a central conduit, as shown, for example, in U.S. Pat. No. 4,157,899. Air is discharged into the filter bags through a series of outlets in the bottom of the rotating arm. This arrangement is not ideal, however. In particular, it is not possible to clean every bag directly below the arm during any one pulse of air because of the manner in which the bags are geometrically distributed beneath the arm. Advocates of this arrangement point out that with multiple passes of the arm, and with pulse timing adjusted so that pulses are not directed at the same point on each rotation, statistically and over some period of time almost every bag will be pulsed. Still, however, many bags are not directly pulsed--i.e., a pulse of air is not directed down through the center of the bag--and the overall efficiency of cleaning therefore is significantly less than what it could be and what would be desired.